The present invention relates to a method of two-way radiocommunication between fixed stations and mobile stations using on the one hand a set of communication channels a first group of which forming a "down" path is reserved for transmission from all the fixed stations and a second group of which forming an "up" path is reserved for transmission from all the mobile stations, each pair of channels formed by one of the communication channels on the "up" path and by one of the communication channels on the "down" path being capable of handling a two-way communication between any one of the fixed stations and any one of the mobile stations and on the other hand at least one semaphore channel used to provide control of communications.
In general, the invention is applicable to two known methods of radiocommunication: the time-division multiple-access type, called TDMA, and the frequency-division multiple-access type, called FDMA. In accordance with the classical technique, these methods are used in a cellular network, divided geographically into patterns of elementary zones, called cells. Each pattern comprises a predetermined number of cells, for example equal to 7, 9, 12 or 16 cells. The patterns are regular and periodically distributed. Each cell is generally hexagonal and has a fixed station at its centre.
In accordance with the radiocommunication method of the FDMA type, for example that described in the French patent application No. 2 376 570, the transmission means of the fixed station of each cell are narrow-band transmitters which are equal in number to the maximum number of mobile stations able to communicate with the fixed station. In each pattern the frequency bands allocated respectively to the fixed stations are different in order to avoid mutual interference between stations in the pattern. Each channel, having a different frequency from that of all the other channels of the pattern in which the fixed station is included, is thus assigned to a communication between a fixed station and a mobile station. A dynamic frequency allocation may also be provided for. It consists in placing certain channels at the disposital of all the cells; when a cell uses one of these channels, that channel cannot be used by the other stations of the same pattern. This is described in the European patent application No. 0 036 146.
In accordance with the radiocommunication method of the TDMA type, the transmitters of the fixed stations of the cells in each pattern are wide-band transmitters. Each transmitter of a fixed station is allocated certain time intervals of a numerical frame equal in number to the maximum number of mobile stations able to communicate with the fixed station. Each communication between the fixed station and a mobile station thus occupies a time interval of the frame which is allocated during the entire communication. A dynamic time allocation of certain time intervals may also be provided for in a manner similar to the dynamic allocation of the FDMA type.
Mention should also be made of a mixed radiocommunication method TDMA-FDMA which consists in allocating to each fixed station a frequency band in which a numerical frame makes it possible to effect connections with all the mobile stations of the cell. This method will not, however, be considered in the context of the present invention.
The communications being effected in the duplex mode, i.e. in both directions simultaneously, the mobile stations have transmission means similar to those of the three methods described above.
In accordance with the three methods, the division of the fixed stations into cellular patterns is mainly necessitated by the phenomenon of interference which prohibits the re-utilization by a neighbouring cell of a radio telephone channel already assigned to another cell. The weakness of these three methods is poor utilization of the frequency spectrum: if one has, for example, a pattern of order 7, each cell only utilises one-seventh of the available frequencies. The capacity of the system might be increased by reducing the size of the cells, but this increases their number and hence the cost of the system as a whole.
The aim of the present invention is to remedy this drawback.